The present invention relates to a technique of depth dependent analysis in thin films using optical or other radiation, on depth scales of ca. 1 to several thousand micrometers.
The capacity to reliably analyze and to recover images of the depth dependent properties of a thin film, which the invention specified in this application discloses, is of fundamental importance to both industrial film processing, materials science and to medicine.
Many processes employed in the fabrication of thin films for industrial applications involve or produce a depth variation of the material composition on the length scale of a few micrometers to a few millimeters. Many of the present day coatings systems used by industry consist of four or five layers or more, for example. The migration of additives such as plasticizers and stabilizers, through thin films is a commonly encountered problem, as is the problem of thermal and optical degradation, which directly affect film performance and lifetime.
In histology, the depth variation of tissue properties on micrometer length scales may be fundamental in understanding tissue function, assessing drug delivery, or in diagnosing disease.
While the problem of analyzing material composition with depth on these length scales is of great interest to a number of fields, relatively few methods exist to achieve this analysis both easily and reliably. This is true regardless of whether or not the analysis method is destructive of the material under study.
Past destructive depth dependent analysis methods for films have involved lateral (orthogonal to the depth axis) stripping or microtomy of thin layers from the original test material followed by chemical or optical analysis of the sampled layers. See the article by A. P. Aleksandrov, V. N. Genkin and V. V. Sokolov, Polym. Sci. USSR 27, 1188 (1985).
Primary difficulties with the above destructive sampling methods are the time and labor required by the stripping or microtomy procedure. The depth sampling is not always reliable: it is not always possible to ensure that layers of precisely equal thickness have been sampled, leading to calibration difficulties in expressing material composition as a function of depth. A strict conformity of the sample to a solely one dimensional (depth) variation of properties must usually apply. The number of depth samples that can be recovered by these methods is usually relatively small. Finally, the microsampling procedure itself may modify the sample itself or contribute depth dependent contamination of the sampled material.
A common destructive analytical method described in the article by J. L. Gardette, S. Gaumet, and J. L. Phillippart, J. Appl. Polym. Sci. 48, 1885 (1993) is associated with conventional light microscopy, and has been used for the depth analysis of polymeric materials. This latter preparation procedure involves embedding the test material in a matrix of resin which acts as a substrate for the cutting of thin cross-sectional slices (orthogonal to the depth axis) of said material, using a microtome apparatus. The thin cross-sectional slices which are cut from the material are then analyzed by transmission or reflectance microscopy.
The difficulties encountered with this procedure are numerous. The primary setting of the sample in the matrix is time consuming, and the, use of a microtome apparatus, while routine in many laboratories, is an expensive requirement of the sample preparation procedure. Many materials have weak adhesion to the resin substrate in which the test material is embedded. Individual layers comprising the material tend to easily delaminate under slicing by the microtome blade. This produces an obvious violation of the mechanical integrity of the original sample, and may seriously complicate the interpretation of the experimental micrographs.
As a result of the above complications, there may be many practical situations in which a destructive depth resolved analysis of a test material on the 1-100 micrometer length scale is not possible.
The above difficulties with destructive sampling methods have led to the more recent development of non-destructive methods of depth profile analysis, usually based on the interaction of optical radiation with the test material. An effectively comprehensive list of these methods consists of the following methods: (i) photoacoustic and photothermal spectroscopy (see the article by R. J. W. Hodgson, J. Appl. Phys. 76, 7524 (1994)); (ii) attenuated total reflectance (ATR) (see the article by R. Shick, J. L. Koenig, and H. Ishida, App. Spec. 50, 1082 (1996)) and variable angle reflectance methods; (iii) optical computed tomography (see the article by S. Kawata, O. Nakamura and S. Minami, J. Opt. Soc. Am. A 4. 292 (1987)); (iv) methods which integrate the material under analysis into the cladding of an optical waveguide (see the article by P. W. Bohn, Anal. Chem. 57, 1203 (1985)); and (v) techniques of confocal microscopy (see the article by T. Wilson and C. Sheppard, Theory and Practice of Scanning Optical Microscopy, Academic Press, London, 1984).
With the exception of confocal microscopy (as discussed in more detail below), the above methods are based on indirect depth detection mechanisms. In these cases, the experimental detector response is measured as a function of some depth sensitive parameter or condition in the experiment, and a depth profile of the sample properties is then recovered from a mathematical analysis of the detector data. The mathematical problem of reconstructing a depth profile of the sample properties from the experimental data in most of these cases, requires application of an inverse scattering theory. The reconstruction problem is usually very ill posed, which means that the experimentally measured signals have only a weak dependence on the depth of an optically interacting feature. In practical terms, ill posedness requires that the data being analyzed must be highly free of both systematic and random errors if the reconstructed depth profile is to be reliable.
For example, optical depth profiling methods based on photoacoustic and photothermal spectroscopy measure signals arising from transient or modulated heat flow in the test material. This heat flow in turn arises from light absorption as a function of depth in the sample, caused by irradiation of the sample with a pulsed or modulated optical beam. The measured photothermal or photoacoustic signal derives its depth sensitivity from the signal""s dependence on the optical beam""s modulation frequency (or, in the case of pulsed irradiation, on the delay time past application of a short irradiating impulse). This signal dependence is mathematically related to the depth of an absorbing feature below the surface. Reconstruction of a depth profile of optical absorption from photoacoustic or photothermal signals has been experimentally demonstrated, but to date, this can only be done if the sample is substantially planar, having a variation in structure along the thinnest dimension, which are called herein the depth dimension, and substantial homogeneity along all directions transverse thereto. Materials for optical photoacoustic or photothermal depth profile analysis must furthermore be substantially homogeneous in their thermal properties, and measurements must be carried out under conditions of a precise knowledge of the sample""s detection geometry. Relative errors in the experimental data must be less than 1% of the full scale signal, typically, for a reliable depth profile reconstruction.
A related set of depth profiling techniques based on attenuated total reflectance (ATR) of an optical beam, measure depth dependent optical absorption in the test material by launching evanescent optical waves into the material. This is accomplished by means of a slab or guide of optical material of large refractive index which is physically contacted to the material under test. By varying the launch angle of radiation entering the slab, the depth of penetration of the evanescent wave into the test material is varied, ultimately causing a variation in the radiation intensity leaving the slab. A mathematical relationship has been derived between the absorption depth profile of an arbitrarily layered planar material and the launch angle dependence of the radiation intensity transmitted by the optical slab or guide. Mathematical procedures, based on inverse scattering theory, for recovering a depth profile of optical absorption from the experimental data have also been published. However, this depth profiling method has many problems in common with the photoacoustic photothermal depth profiling method described above, including the requirement for high quality experimental data and a precise control of all experimental errors. The sample geometry must be carefully controlled and data of very high precision must be available to obtain a meaningful depth profile. Finally, the optimum performance for this method is restricted to a depth range of less than ten micrometers, which is not convenient for many practical problems.
A class of depth profiling methods related to those based on ATR, use integration of the test material into the cladding of a planar optical waveguide. By varying the launch angle of an optical beam into the waveguide, individual waveguide modes of varying order (index) are excited. These mode fields penetrate the cladding (which the test material comprises) to a variable extent depending on the mode field index. The cladding material may luminesce, absorb or scatter the excitation light, and will do so to a depth variable extent depending on the spatial distribution of the electric field excited in the guide for a particular waveguide mode. A known mathematical relationship exists between the mode field index and emission depth profiles of arbitrary dependencies for planar structures, but like the ATR problem, this method is extremely ill posed and highly indirect. It requires even more stringent controls on experimental conditions to recover a reliable depth profile from the test material.
More recent optical depth profiling methods have been advanced based on optical computed tomography (OCT). This method involves optical irradiation of the sample from many directions with measurement of an image of the sample for each direction of irradiation. A volume distribution of optical absorption, scattering or photoluminescence may then be reconstructed mathematically from an optical model of the ray propagation (or diffraction) as a function of irradiation angle. This problem, as with methods (i)-(iv) above, is indirect, and ill posed, requiring the stable solution of an inverse scattering theory for reconstruction of the volume distribution.
Confocal microscopy is a more direct method for recovering depth dependent images of materials. This method derives its depth dependence from the precisely controlled conjugate relationship which is experimentally maintained in the microscope system, between the measured image and an individual slice plane of an irradiated depth dependent object. This relation strongly rejects out-of-focus light contributions in the image, which originate from object planes above or below a precisely defined conjugate plane of interest.
This method has a greater potential depth discrimination for images individually based on light absorption, photoluminescence and light scattering by the object. A one dimensional planar geometry is not required. However, this method is still indirect, and derives its depth dependence from an assumed value of the refractive Index of the sample. Depth position calibration of the measured images may be affected by refractive index gradients in the sample, both along the axis of the microscope and transverse to it. This may prove to be a serious disadvantage in the analysis of complex materials.
It is therefore an object of the invention to provide an improved method and apparatus for analyzing depth dependent properties of thin films, coatings or thin materials. The improved method and apparatus leads to a directly obtained image of radiation interaction as a function of layer depth without destructive sampling of the depth region of interest. The improved method and apparatus also exhibit reduced perturbation by the index of refraction.
According to one aspect of the invention, there is provided a method and apparatus for measuring depth dependent optical absorption, photoluminescence and light scattering in a sample with depth dependent optical properties. This method of depth profile measurement has a number of advantages over the prior art, which are discussed in detail below. These advantages consist of directness of the detection mechanism, simplicity of the apparatus and measurement principles, simplicity of sample preparation and ease of calibration of the length scales involved in the measurement. No mathematical processing of the image is required for direct qualitative inspection of absorbing and scattering features. In addition, this new method of optical measurement may provide several possible mechanisms of image contrast simultaneously, including some new contrast mechanisms. The prior art techniques usually implement one contrast mechanism at a time.
According to the invention, there is provided a method of analyzing composition or structure of a thin film or layer by measuring a depth dependent profile of at least one of absorption, photoluminescence, secondary chemi-luminescence, black-body emission and scattering of radiation in the thin film or layer. The method comprises providing a suitable test material having a depth axis extending through a thin film or layer, the test material being provided with a substantially flat image transfer surface having an orientation substantially parallel to and in proximity to the depth axis, projecting a beam from a source of radiation into the test material along the depth axis thereby causing emission of radiation from the test through the image transfer surface; and collecting at least part of the emission of radiation transmitted through the image transfer surface to form an image of a depth dependent profile of at least a part of the thin film or layer.
According to another aspect of the invention, there is provided an apparatus for light profile microscopy comprising the following elements:
a radiation source which provides a collimated radiation beam;
a suitable test material through which the collimated radiation beam propagates along an axis called the depth axis, and in which test material, the collimated radiation beam irradiates the test material in the volume intersected by the collimated beam and the test material, the volume being called the irradiated volume of the test material, wherein radiation emission occurs from the irradiated volume of the test material by scattering or luminescence;
a substantially flat surface of the suitable test material, called the image transfer surface, the surface having orientation parallel to the depth axis, and through which the surface the radiation emission from the irradiated volume of the test material is transmitted;
a imaging system aligned with its principal axis oriented perpendicular to the image transfer surface, and also aligned so that the principal axis intersects the irradiated volume, the imaging system being aligned also so as to form an image of the irradiated volume from radiation emitted from the irradiated volume and transmitted through the image transfer surface of the test material;
a camera or image recording device which records the image formed by the imaging system.